This invention relates to processes and apparatus for recovering sulfur. In particular, it relates to removing vaporized elemental sulfur from gaseous streams, such as, from the tail gas of a Claus process.
The Claus process is widely used by the industry for the production of elemental sulfur. The process is designed to carry out the Claus reaction: ##STR1## The reaction is favored by decreasing the temperature and by the removal of vaporized elemental sulfur.
In the conventional Claus process, the operating conditions of the reactors in which the Claus reaction is carried out are selected to maintain elemental sulfur in the vapor state. Otherwise, the elemental sulfur would deposit on the catalyst and deactivate it. To assure high conversion rates, the reaction is carried out in two or more consecutive reactors. Elemental sulfur is condensed and removed from the effluent of the intermediate ractors before it is passed to the subsequent reactor. The removal of sulfur allows maintaining the reactors at progressively reduced temperatures. Generally, the initial reactor is operated at about 550.degree.-650.degree. F. (228.degree.-343.degree. C.); the second reactor is operated at about 450.degree.-500.degree. F. (232.degree.-260.degree. C.); and the third reactor is operated at about 400.degree.-420.degree. F. (204.degree.-216.degree. C.). About 85% of the available sulfur is recovered in the condensers upstream and downstream of the first reactor. In the condenser following the second reactor the overall sulfur recovery rate is increased to about 94% and in the condenser following the third reactor the overall sulfur recovery rate is increased to about 96%. The effluent from the third reactor still contains, therefore, about 4% of the originally present sulfur compounds. The value of additionally produced sulfur generally does not justify the use of more than two or three reactors. Accordingly, the conventional Claus process generally removes about 94-96% of the originally present sulfur.
It is highly desirable to modify the Claus process to recover a higher percentage of sulfur for two reasons. First, the presence of sulfur and sulfur compounds in the process effluent necessitates treatment so as to minimize the pollution problems. Generally, the effluent is passed to an incinerator and the resulting tail gases are then discharged through a stack. The treatment of tail gas to eliminate sulfur by processes, such as the Scot or the Beavon process, is expensive and consumes valuable energy.
Second, further recovery of sulfur in the elemental form improves the overall recovery of elemental sulfur and therefore improves the economics of the process.
One approach for increasing sulfur recovery in a Claus plant is to reduce the concentration of uncondensed sulfur in the final condenser effluent. Many plants have final condensers which were installed several years ago and operate with effluent gas temperatures as high as 300.degree. F., which results in a concentration of about 0.2% by weight of elemental sulfur vapor in the effluent gas. The final condensers in these plants have not been converted to a more efficient design because the cost using designs heretofore available would be greater than the benefit. Some newer and larger plants have condensers with an effluent gas temperature in the range of 250.degree.-270.degree. F. (121.degree.-132.degree. C.), which reduces the concentration of elemental sulfur vapor to about 0.03-0.06% by weight. Even this lower loss rate is objectionable in these plants but heretofore it has not been feasible to cool to temperatures below about 250.degree. F. (121.degree. C.) because solid sulfur would form and cause plugging of equipment. An inexpensive method for overcoming this freezing point limitation has been needed.
The other approach toward increased recovery of elemental sulfur in a Claus plant is to increase the conversion of hydrogen sulfide and sulfur dioxide to sulfur. One method for increasing the conversion is to use a low temperature catalytic reactor, for example a cold bed adsorption (CBA) reactor, to treat the effluent from the final reactor of the Claus process. The reaction is generally carried out at a temperature range from about 250.degree.-280.degree. F. (121.degree.-138.degree. C.) which results in the condensation of elemental sulfur on the alumina catalyst. The low temperatures in the CBA reactor favor the reaction and the condensation of sulfur removes it from the reaction phase thereby allowing more H.sub.2 S and SO.sub.2 to react. The use of a CBA reactor with the Claus process can bring the overall recovery of elemental sulfur to about 99%.
There is a limitation on the low temperature at which the low temperature catalytic reactor can be operated. The gas from the final conventional Claus reactor is cooled in the sulfur condenser before entering the low temperature reactor. Heretofore it has not been feasible to cool the gas to below about 250.degree. F. because plugging of the condenser would result. A method for cooling to lower temperature has been needed in order to increase the recovery potential of the CBA process.
There is therefore a long felt and still unsatisfied need for a process that would make it feasible to cool gas streams in the Claus process to temperatures below the point where sulfur solidifies; such a method would increase the overall recovery of sulfur to levels which were heretofore not achievable and would eliminate or reduce the amount of anti-pollution equipment used in the sulfur recovery process. The present invention achieves the above-stated goal with a process which utilizes simple, reliable and inexpensive equipment that can be manufactured from commercially available parts.